Saab's fab 5: New engine is smooth, strong

At Saab Automobile AB in Sweden, engineers are waiting for a go-ahead to build a five-cylinder 'variable compression' engine that uses half the displacement of its current V-6, has more power, and burns 20 percent to 30 percent less fuel.

Saab has shown the technology at auto shows in Europe and North America this year and has demonstrated the engine on the road in its big 9-5 sedans.

The birth of the small five-cylinder package is a story of engineering tinkering and experimentation.

Saab engineers hit on the idea of variable compression almost 20 years ago. Saab envisioned a cylinder head that literally moved up and down with integrated cylinders.

The change in head position would increase or decrease the volume of the combustion chamber.

That effectively would turn a small engine into a larger one when more power was needed and back again for normal driving.

When it first moved the concept into prototype in 1989, Saab assumed it would need to package it as an inline six-cylinder engine. But it discovered that a six-cylinder architecture would be about 3 inches too long.

So in the early 1990s, engineers eliminated one cylinder.

But five-cylinder engines usually beg for counterbalancing to solve their vibration problems. Saab's engine concept had no room for balance shafts.

The necessary space on the engine's backside was now occupied by the hinge hardware that enabled the head to move up and down.

But instead, Saab relied on a small displacement to dampen the vibration.

At just 1.6 liters, the prototype is inherently smooth, according to the company.

Smoother, in fact, than Saab's four-cylinder engines - even without balance shafts.

According to Saab, any four-cylinder engine will vibrate as its pistons seesaw, two pistons up and two down at any given time. Because Saab's engine is of a low enough displacement that it does not shake much to begin with, the fifth cylinder serves as a sort of internal counterbalance.

For Saab, the result is a 1.6-liter engine that delivers 225 hp and 224 pounds-feet of torque.

By comparison, the current 3.0-liter Saab V-6 delivers 200 hp and 229 pounds-feet of torque.